Operational amplifiers for integrated circuits, such as for example the amplifier described by G. M. Jacobs et al. in "TOUCH-TONE Decoder Chip Mates Analog Filters with Digital Logic", Electronics, pp. 112, FIG. 7, Feb. 15, 1979, commonly occur in large numbers in a single integrated circuit. Therefore, it is important that their size be kept to a practical minimum, in order to keep the required chip area to a minimum. The necessity of maintaining a small size for the amplifiers often makes it particularly difficult to design into them certain desirable operating characteristics for a given circuit application.
One generally desirable operating characteristic of operational amplifiers used in integrated circuits for communication carrier applications is a high settling speed of the response for a step input signal. For small signals, this settling speed is a function of the bandwidth and the stability margins of the amplifier, which in turn are generally determined by fixed circuit parameters as demanded by the particular application of the circuit. For large signal steps, on the other hand, the settling speed is limited primarily by the slew rates of the amplifier. The slew rate, positive or negative, is the maximum rate at which the output voltage can change in the positive and negative direction, respectively, and for integrated-circuit operational amplifiers is commonly expressed in units of volts per microsecond. The expression "large signal" as used herein refers to a signal of at least such an amplitude that the linearity of the amplifier begins to be strained by the slew rate limitations. Thus, for an amplifier with given constraints of bandwidth and stability margins, an increased slew rate provides an avenue for improving the settling speed for large step input signals. While it would be possible to increase the slew rate by the direct method of increasing the size of the output transistors, such a step would require significantly more area for the amplifier and be likely to result in significantly increased size or power consumption.